The human intestine is thought to be sterile in utero, but it is exposed to a large variety of maternal and environmental microbes immediately after birth. Thereafter, a dynamic period of microbial colonization and succession occurs, which is influenced by factors such as delivery mode, environment, diet and host genotype, all of which impact upon the composition of the gut microbiota, particularly during early life. Subsequently, the microbiota stabilizes and becomes adult-like [1]. The human gut microbiota contains more than 1500 different phylotypes dominated in abundance levels by two major bacterial divisions (phyla), the Bacteroidetes and the Firmicutes [2]. The successful symbiotic relationships arising from bacterial colonization of the human gut have yielded a wide variety of metabolic, structural, protective and other beneficial functions. The enhanced metabolic activities of the colonized gut ensure that otherwise indigestible dietary components are degraded with release of by-products providing an important nutrient source for the host and additional health benefits. Similarly, the immunological importance of the gut microbiota is well-recognized and is exemplified in germfree animals which have an impaired immune system that is functionally reconstituted following the introduction of commensal bacteria [3-5].
Dramatic changes in microbiota composition have been documented in gastrointestinal disorders such as inflammatory bowel disease (IBD). For example, the levels of Clostridium cluster XIVa bacteria are reduced in IBD subjects whilst numbers of E. coli are increased, suggesting a shift in the balance of symbionts and pathobionts within the gut [6-9,18].
In recognition of the potential positive effect that certain bacterial strains may have on the animal gut, various strains have been proposed for use in the treatment of various diseases (see, for example, [10-13]). A number of strains, including mostly Lactobacillus and Bifidobacterium strains, have been proposed for use in treating various bowel disorders (see [14] for a review and see [15]). Reference [16] proposes the use of strains of the genus Blautia for use in modulating the microbial balance of the digestive ecosystem. In the context of reference 16, modulation refers to promoting the activity of the acetogenic bacterial flora to the detriment of methanogenic and sulfur reducing bacteria. This document therefore teaches only an increase in acetogenic bacteria. There is no discussion relating to the diversity of species belonging to a number of taxa. Reference [17] discusses the use of Blautia for improving survival in patients affected by graft versus host disease (GVDH). It mentions that increased bacterial diversity is associated with reduced GVDH-related mortality and that increased amounts of bacteria of the Blautia genus were associated with reduced GVDH. However, there is no suggestion that the administration of Blautia to a patient effects an increase in microbiota diversity and/or induces stability of the microbiota in a subject.
The relationship between different bacterial strains and different diseases, and the precise effects of particular bacterial strains on the gut and at a systemic level and on any particular types of diseases, are poorly characterised and results to date are variable and pose more questions than provide answers [18].
A hallmark of many human diseases linked to microbiota alteration is loss of microbiota diversity. As distinct from so-called dysbiosis which is simply an altered microbiota composition compared to the typical aggregate microbiota in healthy subjects, loss of microbiota diversity may be quantified by a measurable reduction in number of the sequence-based bacterial classifications or Operational Taxonomic Units (OTUs) in a sample, typically determined by 16S rRNA amplicon sequencing methods. Loss of diversity is also measured by reductions in the Shannon Diversity Index, or the Chao index. Reduced microbiota diversity is reported in recent studies of obesity, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), type 2 diabetes and frailer older people [20]. Re-establishing the healthy microbiota can be difficult as the bacteria in the gut are resistant to colonisation. This poses a challenge when trying to treat the microbiota of unhealthy subjects by increasing the diversity of the microbiota [19]. The accompanying loss of microbial metabolic function is assumed to be a contributory factor to the symptoms of these pathophysiologies. In contrast to healthy adults in whom the microbiota is stable, the microbiota of unhealthy subjects such as those suffering IBD, IBS and frail elderly subjects is unstable [18, 20].
There is a requirement for the potential effects of gut bacteria to be characterised so that new therapies using gut bacteria can be developed.